120 THE STORY OF FISH LIFE. 



young have attained their full development. At 

 this period both sexes aie alike, having very 

 small teeth, and feeble jaws. In April, however, 

 the male loces his small teeth and replaces them 

 by very long and strong teeth, and with the 

 advent of these new teeth increases the size of 

 the jaws. The teeth of his mate remain un- 

 changed. July and August sees the death of all 

 the adults, so that in September only young fry 

 are to be found. Thus in this goby we have at 

 least one instance of an annual vertebrate. The 

 fifteen-spined stickle-back (Gasterosteus spinachia) 

 is said likewise to run its life's race in the course 

 of a single year. 



Some other fish appear to spawn but once and 

 then die, but these take more than one year to 

 come to maturity. The eels appear to belong to 

 this category. 



CHAPTER X. 



LARVAL FISHES AND THEIR METAMORPHOSES. 



Fishes are born into the world in what is called 

 a larval condition, that is to say, in a condition 

 differing more or less from that of the adult, 

 which is only reached after a series, of frequently 

 well-marked, stages or transformations. Larval 

 vertebrates only occur amongst the fishes and 

 amphibia — the frogs and toads and their kindred : 

 but amongst the invertebrates we have quite a 

 large variety of larvae. The caterpillar is a 

 larval form with which every one must be 



